I've been thinking for a while about klatunickto's problem ("System Keeps Running").

For an air source heat pump, I can see where optimum efficiency is obtained with long run times.  The temperature of the ambient outside air, air from which heat is being 'sucked' out of, stays relatively constant over the duration of each run time.

But this isn't the case for a closed-loop in-the-earth GSHP.  It takes a good amount of time for the heat in the earth to migrate to the adjacent vicinity of the loop pipes.  As the GSHP 'sucks' heat out of the earth near the pipes, the temperature of this earth slowly declines.  Once the GSHP shuts off, heat in the earth then migrates toward the lower temp earth nearest the pipes, but it's a pretty slow process.

So it would seem to me that the efficiency of the system is constantly declining while the GSHP system is running.  And for someone who has to run their GSHP non-stop across an entire day, I would think that the amount of heat available to be 'sucked' out of the earth would become severely depleted.

I'd be curious to hear from the experts if indeed this is a phenomena associated with closed-loop in-the-earth GSHP systems.  And if so, then why would the capacity for this type of system be optimally designed to have long run times.

Below is a chart to illustrate what I'm saying.  It's an illustration of what I'm observing with my closed-loop (vertical) GSHP system.

The x-axis is the undisturbed 'down-deep' earth temp - the constant year-round temp of the earth if there were no pipes present.  In my case, this is 68°.

The red line is the rate of heat being 'sucked' out of the earth, in KBTU/hr.  Since the x-axis represents time, the area then between the red line and the x-axis is the amount of heat being 'sucked' out of the earth.  Longer run times widen the area.  First vs. 2nd stage lengthens the area.

The blue and black lines are EWT for the 3 and 5 ton GSHP units I have, respetively.  The black line shows that, given resonable 'rest' time between cycles, my 'down deep' earth temp near the pipes is down about 1° - i.e., it's currently at about 67°.

As the intensity of heat is 'sucked' from the earth (more frequent cycles, and longer time periods per cycle), the EWT drops, because heat cannot move fast enough in the earth to replace the heat already 'sucked' out nearest the pipes.  Even with the short cycling I have, due to having plenty of compressor capacity, it's not unusual to see EWT drop 2 - 3° across the time periods where heat is being extracted.

And then, particularly in the afternoons when my GSHPs are running less (or not at all), there's enough time for most of the heat lost nearest the pipes, to be replaced.  And the cycle starts over the next 24 hour period.

Looking at this, and trying to imagine what would be the case if my GSHPs ran near continuously (i.e., with smaller ton units and/or a much more leaky residence), it causes me to start questioning what I think is the conventional wisdom of long run times for GSHPs.  It makes sense to me for ASHPs, but not for GSHPs with in-the-earth closed loop 'condensors.'  Isn't the EWT going to keep dropping to some very unreasonable value?

So what part of the picture am I missing?  Or indeed is it advisable to install more tonnage if the heat pump is geo versus air?  And for those that have very long GSHP run times, do they really have a capacity / performance problem with their in-the-earth closed loop?

Many thanks!

Best regards,

Bill

Bill,

Interesting subject.   When designing a closed ground loop,  the idea is not to deliver fluid to the heat pump at or near the deep earth temp.  This could be done in theory  by putting enough pipe in the ground, but is not practical from a cost perspective.

On the slinky job that I just posted pictures for,  I designed that loop for a min EWT of 30­°,  which required 7000' of pipe and 900' of trench.

Had I gone for a min EWT of 40°,  19,000' of pipe and 2400' of trench would have been needed

Had I gone for a min EWT of 45°,  56,000' of pipe and 7200't of trench would have been needed.

If the job had been vertical  for a min EWT of 30°,  1600'  of bore length would have been needed.

For a min EWT of 40°,  2800' of bore length.

For a min EST of 50°.  8150' of bore length.

So it is easy to see why a system can't be built to deliver fluid continuously  at earth temp.

Geo closed loop systems are designed with the idea that the loop temps will drop as the heating season progresses and in a heating dominated climate will get close to 30° at the end of the heating season.

If a house needs 1200 kBtu  in a day,  I don't think it will make any difference in loop temp is this is extracted in large or small time frames.  The loop temp will probably drop at the same rate.

So I think that longer run times will not affect loop temp over time and will pro long the life of the heat pump.

Dewayne, thanks.  Your posting is very helpful.  I didn't know that closed loop systems were designed to get close to 30° by the end of the season, for example.

So does the EWT continuously drop over hours and hours of continuous GSHP operation, all the way down to 30°?

And thus, don't you also have to size the loop/compressors to be able to satisfy the heat demand in a reasonable period of time.

Doesn't the earth in close proximity near the loop pipe need x amount of time to mostly recharge (with heat) in order to keep the EWT from sinking to excessively low levels (assuming KBTU/hr extraction rate is greater than earth heat replenishment rate)?

Thanks!

Best regards,

Bill

Yes...kind of... if you look at this graph of my system,  the red and blue lines are my EWT and LWT,  the green and black lines are my ground temps.

50 days ago (at the beginning of the heating season)  my EWT and ground temps were about the same.  Now there is about a 5° spread.  As the heating season progresses,  my ground temp will drop since this is a horizontal  loop (subject to air temps)  and  the spread between my EWT and ground temp will increase.  My loop is somewhat oversized so it shouldn't get all the way to 32°.


I just started charting COP.  It will be interesting to see how much the COP drops as the EWT drops.

Wow! I worry when my EWT gets down to 40 in February. There must be a better way of transferring the heat to the ground. My main house system has never gotten below 40 for EWT and the console in the garage
has only dropped that low one year and that was the first year of operation (2001) and I think that was before I had the best earth contact.

Heat pumps are sized to run about 100% of the time at design temp which is 8° in Salt Lake.  At this point the loop temp will be approaching the minimum for the year.

The loop design software takes into account the factors you asked about.

As the loop temp drops,  there is a greater difference between the earth temp and loop temp so the heat is pulled over a greater distance through the earth.  You are right... this heat moves very slowly.  On the order of .7- 1.4 BTU per hour per foot.

In colder climates with more HDD,  more pipe is needed in the ground to keep the min EWT above 30°.

In my post above the house is located in Salt Lake City with 6081 HDD.  1600' of bore hole would be needed to keep the min EWT above 30°

The same house in Helena  Montana  with 8190 HDD would need 2550' of borehole to keep the min EWT about 30°.

In theory,  you could get enough pipe in the ground to pull heat continuously from the ground year round and keep the EWT above 30°  when you reach the point where heat is flowing from the ground into the loop  as fast as the heat pump is  taking heat out of the loop.

Bill, one more thing. Since your system is in a cooling dominated climate, your loop will be considerably over sized for heating. I doubt your loop will get any where close to 45° never mind about getting down to 30°

COP loss owing to small drops in EWT (<10 Deg) are fairly minor.

Efficiency and durability improve when short cycles are avoided.

Geo's ability to deliver comfortable warmth evenly distributed is in part derived from longer run times.

Designing a loop field to deliver a average loop temperature of 50 over a year or one that actually is 50 for the year end up with nearly the same operating cost over a year. The loop that provides a average of 50 may start a heating season out at with 90 from the heat stored from the cooling season. The heating season will start with super high COPs and end up with 30 at the end of the winter starting the summer cooling season with supper high EER. In the end the loop will be smaller the best return on investment.

From what I've read here and elsewhere, in-ground carryover of heat from summer cooling long enough for winter heating (and conversely) has been largely discounted. Still, the idea is so attractive that I'm open to data to the contrary...

The length of the shoulder season (limited heating or cooling) combined with natural conduction, vertical and horizontal water movement through loop fields all apparently limit the ground's ability to store summer's heat for winter use, and conversely.

This is my observation too.

Below is a chart of one year of EWT for me.  EWT stays between 60 and 80°F.  I don't see any evidence that I'm starting this Winter with warmer water than I did last Winter.

For me, though, a conclusion like this is probably better made next year, a year from now.  I'm probably gaining some benefit now with earth having had about 18 months to settlie in around my 4800' of vertical bore length.  And thus, for this Winter only, I may see a little warmer water than last year.  We'll see.

Best regards,

Bill

Dewayne, I really appreciate the effort you put in to reponding in this thread - thanks.  I've spent some time studying your comments.

I think I pretty well understand the long term design aspect of a closed loop system - particularly the trade-off between the amount of material put into the ground and installation of it (cost) versus how low the EWT will go before the earth has enough opportunity to replace the heat nearest the loop pipe.  Looks like each of the major geo HP manufacturers makes available s/w that makes the calculations speedy, accurate, and specific to the location.  Thanks for the examples.

I'm not sure I yet understand the real-time dynamics of EWT, but, I'll pay attention here and learn over time from the experts.  There must be some amount of heat, in the loop water, that's initially picked up, before the earth starts suppying heat to the pipe.  Thus, as I see in my case, there's an initial drop of a few degrees for EWT, to steady state of heat extraction versus heat transfer through the earth.

Actually, I suspect that EWT, after the first few hour significant drop, would continuously and slowly decline, noting how slow the heat moves through the earth.  Those who have their system running all of the time are going to see the long term EWT drop faster than those who have short cycle times.  I think this is illustrated below for my instantaneous EWT at the moment.

I looked at the WaterFurnace specs for my 5 ton unit in 1st stage.  Looks like there's an average of about a 14% drop in Heat Extraction (KBTU/hr) ability for every 10° drop in EWT.  I.e,

20° - 18 KBTU/hr
30° - 24 KBTU/hr
40° - 29 KBTU/hr
50° - 34 KBTU/hr
60° - 38 KBTU/hr
70° - 43 KBTU/hr
80° - 47 KBTU/hr
90° - 51 KBTU/hr

So if the water starts out at 70° at the beginning of the heating season, the unit has to run not quite twice as long once the water's down to 30°, to put the same amount of heat into the structure.

Many thanks to everyone who has contributed to a good learning thread.

Best regards,

Bill